1. FIELD OF THE INVENTION
The invention relates to the use of a phase shifting mask to improve resolution in the fabrication of sub-micron integrated circuits and more particularly to the fabrication and use of a self aligning double rim phase shifter mask with phase shifting regions, half-tone or partially transmitting regions, opaque regions, and non phase shifting transparent regions.
2. DESCRIPTION OF THE RELATED ART
As optical lithography advances to 0.5 to 0.35 microns and below new technologies are needed to improve the resolution of the imaging lens. Phase-shifting photomasks have been used to improve resolution as well as depth of focus. Phase shifting photomasks are described in a paper "LITHOGRAPHY'S LEADING EDGE, PART 1: PHASE-SHIFT TECHNOLOGY," published in Semiconductor International, February 1992, pages 42-47. The description includes rim type phase shifter in the Table on page 45. This rim type phase shifter uses an opaque material in conjunction with a phase shifting material. Phase shifting photomasks are described in U.S. Pat. No. 5,045,417 to Okamoto.
While improvements have been made in image resolution and depth of field, the linear dimensions used in integrated circuit technology call for even greater improvements in resolution and increased depth of field. This invention uses a double rim type phase shifting mask and uses a partially transmitting material in the mask.